Abstract. Denoising is very important in signal processing. Electrocardiogram (ECG) data always contains 50 or 60 Hz power line interferences (PLI), PLI may affect the detection of P wave, QRS complex and T wave. So the pre-processing of ECG data is necessary. In order to remove PLI, in this paper, we use two improved tracking-differentiator filters (ITDFs), which are signed ITDF1 and ITDF2 respectively. The simulation results for ECG signals obtained from MIT-BIH database indicate that ITDFs are good methods for filtering noises, and ITDF2 does better than ITDF1.